This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 2001-215823, filed Jul. 16, 2001, the entire contents of which are incorporated herein by reference.
1. Field of the Invention
The present invention relates to a large membrane space structure mounted on a spacecraft or space vehicle, and a method for its deployment and expansion.
2. Description of the Related Art
A large membrane space structure means a large membrane structure for use in space, such as a large solar cell module used for obtaining power in space, or a solar sail or photon sail used as a propulsion system in space.
In recent years, there has been an increased demand for exploration of the solar system. A spacecraft such as a so-called rocket, which is propelled by a reaction of high-speed exhaust of combustion gas, can only be loaded with a limited amount of propellant or fuel. Therefore, the search for a new propulsive system that does not need propellant or fuel has been of great interest. Accordingly, the development of a large membrane space structure, such as a solar sail propelled by the reflection of solar radiation, has been strongly investigated.
The large membrane space structure includes a sail to which a membrane is adhered. Aluminum is sputtered onto the membrane and made specula. The sail is deployed and spanned by the centrifugal force owing to a spacecraft or an artificial satellite spin motion. As shown in FIG. 5, the sail 14 reflects solar radiation on the membrane and provides thrust F to a spacecraft or an artificial satellite by means of the reaction caused by light reflection. Some of the large membrane space structures of a practical scale have a rectilinear shape, each side of which may be as long as several tens of meters to a few hundred meters or longer. Accordingly, the membrane is also as large as the structure.
Even the large membrane space structure travels in space where solar gravity acts. Since the light pressure acceleration that acts on the sail 14 is much smaller than the gravity of the sun or the earth, it moves mainly governed by the gravity rather than the thrust F due to the light pressure. More specifically, as shown in FIG. 6, in the solar system, even the large membrane space structure orbits like a planet around the sun. Near the earth, it may orbit around the earth as an artificial satellite.
The thrust F generated by the sail 14 has the function of accelerating or decelerating the orbital motion, or applying acceleration to the space structure in order to change the orbit. When the large membrane space structure starts orbital motion in space, since the acceleration and deceleration are very small, the space structure is gradually accelerated and decelerated.
Referring back to FIG. 5, the thrust F on the planar large membrane space structure where area is A is represented by the following equation:
F=PA(1+r)cos xcex8
where P represents the light pressure of solar radiation per unit area, r represents the light reflectivity of the sail, and xcex8 represents the incident angle spanned by the normal direction of a membrane surface with the direction toward the sun. Since F depends on the steering angle xcex8, if it is assumed that xcex8=0xc2x0 and r=1 that means perfect reflection, the thrust F is represented by the following equation:
F=2PA(N/m2). 
Near the earth, the light pressure P of the solar radiation is very low, i.e., P≈4.6xc3x9710xe2x88x926N/m2. The performance of the large membrane space structure depend on the acceleration. Assuming that the sail 14 is formed of a membrane of an areal density of xcex2 (kg/m2), the mass is represented by xcex2A. If xcex2is 0.01 kg/m2, the acceleration xcex1 is represented by the following equation:
xcex1=2P/xcex2≈9.2xc3x9710xe2x88x924m/s2. 
This is as substantial as the acceleration of an ion engine or a plasma engine.
The acceleration of a large membrane space structure increases with the flight time. Therefore, the more the flight time lingers for the travel, the more advantageous the large membrane space structure is over the chemical engine consuming propellant or fuel.
As shown in FIG. 7, a conventional type of large membrane space structure is rectilinear. The large membrane space structure comprises four spars 32 to spread a sail 30. One end of each spar 32 is supported by a center hub 34. The hub 34 includes a payload and a mechanism for expanding the spars 32 (both are not shown). The attitude of the large membrane space structure may be controlled by the torque generated by tip vanes 36 attached to the tips of the spars 32. The torque may be generated by shifting the center of the pressure of the solar radiation from the mass center of the structure.
When the sail 30 is transported into space, the membrane is folded suitably and may be wrapped around a core material such as a cylindrical pipe, so that it can be packed compactly.
To pack a large membrane space structure having rectilinear membranes, the membranes may be folded and wrapped after the huge sail is produced. However, it is difficult and not practical to carry out this method in a structure of practical scale.
In addition, since the membrane itself is folded and creased, residual stress and strain may be generated and left in the membrane. To smooth out such a fold, a certain spreading force is required. Therefore, the fold is the most crucial factor that prevents the sail from being deployed in space. Otherwise, since a number of complex structures are required to deploy the sail, the deployment may even be unsuccessful.
Moreover, the sail of the large membrane space structure may require an outer frame. For example, it is sometimes assumed that framework members, such as expandable spars, are used to spread the sail. Since the framework members must be very large and stiff, the mass thereof cannot be reduced easily. Therefore, this may result in the considerably large vehicle required to transport the large membrane space structure into space.
Furthermore, in the large membrane space structure made of a single sail, since the amount of torque applied to the very large structure cannot be controlled easily, it is difficult to adjust the rotation speed of the spacecraft.
The present invention was devised to solve the above problems, and an object thereof is to provide a large membrane space structure and a method for its deployment and expansion.
To solve the above problems, according to an aspect of the present invention, there is provided a large membrane space structure mounted on a spacecraft comprising:
a) a hub including:
a plurality of supports, with a first imaginary fulcrum at the center of the hub, a first support member which is stiff, a second support member which is a beam structure that may be hinged on at least a midpoint thereof, and first rigging connecting ends of the first and second support members as well as the hub; and
control means for deflecting the supports at desired angles with respect to the spacecraft by rotating them about an imaginary center line extending through the first fulcrum and the midpoint of the second support member as a pivotal member; and
b) a sail including petals that are symmetrical with respect to the first fulcrum when deployed and attached to the supports, each petal including:
membranes spanned on first regions symmetric with respect to the imaginary center line and including the first fulcrum, a second fulcrum located on the imaginary center line and separated from the first fulcrum, and two points symmetric with respect to the imaginary center line, the membranes spanned on second regions defined by a peripheral portion of the first region opposite to the second fulcrum and a plurality of split lines extending from the second fulcrum to the peripheral portion at arbitral intervals; and
bridge belts along the split lines to the peripheral portion discretely connecting the membrane elements to one another at intersections between split lines and a plurality of imaginary lines extending from an end of the second support member to an end portion of an outermost membrane elements opposite to the first fulcrum, the bridge belts providing tension across the membranes.
According to another aspect of the present invention, there is provided a method for deploying and spanning the large membrane space structure recited in claim 1, in which the petal has folds in the bridge belts, and which is folded such that adjacent membranes are faced each other, and is wrapped and packed around the hub, the method comprising:
rotating the petal in a predetermined direction about the first supporting point;
extending first the petal radially from the hub by centrifugal force generated in radial directions perpendicular to a direction of rotation of the petal, thereby unwrapping the membrane elements from the hub by tension generated in the radial directions while the membrane members are folded at bridge lines, and rotating the support and the petal about the imaginary center line at a desired angle; and
unfolding the folds by tension acting on the bridge belts by the centrifugal force, and deployment force in the circumferential direction of the petal generated by both the centrifugal force and tension supporting lines extending from the end of the second support member at certain obliged angles with respect to a radial direction of the centrifugal force, thereby deploying the membrane elements.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter.